Roller cone drill bits for drilling earth formations conventionally have roller cones (which may also be characterized as rolling cutters), each cone mounted on a head or head on a distal end of a leg protruding from or secured to a bit. As the drill bit rotates to drill an earth formation under applied weight on bit (WOB), the roller cones are caused to rotate on their respective heads responsive to WOB and engagement with the formation. The outer surfaces of the cones include cutting structures, which are conventionally in the form of teeth formed integrally with the roller cones or inserts disposed in recesses formed in the outer surfaces of the roller cones. The cutting structures impact, gouge and crush the underlying earth formation material as the drill bit is rotated within the wellbore to form the borehole.
A bearing system is included between each cone and its associated head. The bearing system may include a thrust bearing and a radial bearing. The bearings of the bearing systems are located and configured to carry the load to which the roller cones are subjected while the bit is rotated under WOB, while allowing the roller cones to rotate on the heads. Specifically, the thrust bearing carries the axial component of the load applied parallel to a longitudinal axis of the head, while the radial bearing carries the radial component of the load applied perpendicular to the longitudinal axis of the head. The high loads present during drilling cause friction in the rotating components and generate heat, which may cause deterioration of the bearings. Bearing deterioration may cause bit failure, resulting in time-consuming and expensive removal and replacement of the bit from the borehole.
Bearing systems used in roller cone bits may be sealed bearings or open bearings. Sealed bearing systems conventionally include a lubricant reservoir for supplying lubricant, such as a bearing grease, to the bearing surfaces between the roller cones, the heads and the bearing elements. A pressure compensator may be used to equalize the lubricant pressure with the fluid pressure within the borehole to prevent pressurized borehole fluids from invading the lubricated volumes between the head, cone and bearings. Open bearing systems, by contrast, have no seals or bearing grease. Open bearing systems may use drilling fluid, such as a drilling mud, to both cool and lubricate the bearings.
Additionally, a small amount of clearance (e.g., on the order of a few thousandths of an inch) is conventionally provided between mating components of the cones and their corresponding heads due to manufacturing and assembly constraints. This clearance enables a small amount of movement or “play” between the cones and their corresponding heads in both the axial direction parallel to the longitudinal axis of the head and in the radial direction perpendicular to the longitudinal axis of the head. This play between the cones and heads may result in misalignment between the bearing system components during drilling, which may result in excessive wear of the misaligned bearing components.
Recently, new connection methods for connecting roller cones to corresponding heads of a bit body have been developed in which the cones are retained on their corresponding heads using tensioner bolts, such that the tensioner bolts apply forces urging the cones onto their heads to substantially eliminate play between the cones and the heads. Examples of such connection methods are disclosed in U.S. Pat. No. 8,950,515, issued Feb. 10, 2015 to Schroder, and in U.S. Pat. No. 8,955,622, issued Feb. 17, 2015 to Schroder, the disclosure of each of which is hereby incorporated herein in its entirety by this reference.